The invention relates to a subassembly consisting of a drive shaft, a housing with an aperture through which the drive shaft projects out of the housing, and an axial face seal comprising a sliding ring and a counter-ring, which creates a seal between the components drive shaft and housing. Expressed more generally, the invention relates to the sealing of shaft exit apertures by means of a seal consisting substantially of a sliding ring and a counter-ring against which the sliding ring is pressed in the axial direction of the shaft. The invention relates in particular to an axial piston compressor in which the housing is closed by a cover plate within which is disposed the exit aperture for the drive shaft as well as an axial face seal for the drive shaft. Such a compressor, with a subassembly of the kind described above, is known from the patent EP 0 864 787 A2.
In the following the invention is described with reference to an exemplary application for an axial piston compressor such as can be used in particular in an air-conditioning unit for motor vehicles. However, as explained at the outset, the invention can be employed in general for machines that comprise an axial face seal.
An axial piston compressor in a vehicle air conditioner serves to suck in a coolant from a heat-transfer compartment, in which it evaporates while taking up heat, and to raise its pressure by compression, so that in another heat-transfer compartment the coolant can release the heat again at a higher temperature level. Subsequently the coolant enters an expansion organ, where it is returned to the pressure level of the first heat-transfer compartment. The function of such axial piston compressors in such a coolant cycle is generally known, so that it need not be explained further here.
An especially important component of an axial piston compressor used in a vehicle air conditioner is the sealing element for the drive shaft. The shaft seal of an open compressor is of particular significance because leakages of the circulating coolant, in particular a coolant leakage through the the shaft seal, has a negative effect on the operating behaviour of the air conditioner by reducing the amount of coolant contained. Furthermore, it is prohibited to allow certain coolants to enter the atmosphere.
As shaft seals in axial piston compressors employing the coolant R134a radial shaft sealing rings are customarily used, because in the presence of the pressure differences encountered there, against which the seal must operate, these rings provide sufficiently reliable sealing at low cost. Such a radial shaft sealing ring is in general inserted into a recess on the outer surface of the housing.
Very recently it has become more common to employ the coolant CO2 as a substitute for the coolant R134a, because CO2 offers many advantages over R134a. However, the coolant CO2 requires a higher pressure level than R134a, so that more technically demanding seals are needed. Therefore it is customary to use axial face seals, in which owing to the cooperation of sliding ring and counter-ring, which are pressed against one another, it is possible to obtain adequate sealing against the high pressure difference. The axial face seal, however, cannot be mounted on the outer surface of the housing, but rather must be disposed in the interior of the housing, for instance behind a cover plate. Furthermore, the relative rotation between the sliding ring and the counter-ring generates so much frictional heat that a controlled dissipation of that heat seems desirable. However, in axial piston compressors for vehicle air conditioners no separate lubricant circulation is provided that could be used to carry away the frictional heat in the region of the sliding ring, nor does the mist of lubricant in the interior of the housing of the axial piston compressor suffice to dispose of the frictional heat produced.
The objective of the invention is thus to provide a shaft seal that can be mounted simply and economically and that enables good dissipation of the frictional heat that is generated.
In a subassembly in accordance with the invention, because the counter-ring is no longer a separate part that must be inserted into the cover plate, less effort is required for installation. Because the counter-ring itself need no longer be so stable that it can be manipulated and mounted as a separate component, it can be made considerably less thick than conventional counter-rings, which for example must be pressed into a recess. The diminished thickness of the counter-ring offers two advantages. First, it reduces the length of the whole structure. Second, it improves the heat dissipation. That is, although such counter-rings are customarily made of material with extremely poor thermal conductivity, because the counter-ring is less thick it can adequately conduct heat away from the component into which it is integrated. The thermal conductivity is also improved in comparison with state-of-the-art constructions in that the counter-ring is now directly connected to the relevant component rather than being partially isolated therefrom, for instance by O-rings that must be pressed into place to serve as seals for the counter-ring according to the state of the art.
The above term xe2x80x9cintegratedxe2x80x9d is used in the sense of this application firstly to designate a unit in which the counter-ring is non-removably connected to the component on which it is seated, and secondly to indicate that said component is so configured that the counter-ring is formed by a section of the component itself.
According to one embodiment of the invention the counter-ring can be glued, welded or soldered to the component that supports it. By this means, with little effort the desired integral and gas-tight connection between counter-ring and supporting component is achieved.
According to another embodiment the counter-ring is formed by a coating applied to the relevant component, for example by means of a CVD or PVD process. This allows the counter-ring to be made particularly thin, so that the axial length of the overall structure is very small. Another result is that the heat conduction from the counter-ring to the supporting component is optimized, because no insulating intermediate layers are present. Still another advantage is that a counter-ring can be obtained, the surface of which needs no further treatment.
The coating is preferably a ceramic layer. A counter-ring so formed is especially resistant to wear and tear. After the ceramic coating has been applied, it is preferably treated mechanically so as to produce the desired low degree of surface roughness.
The ceramic coating preferably consists of SiC. Tungsten carbide is also a suitable material. Then the sliding ring can be made of impregnated hard carbide, which has proved an advantageous pairing with respect to friction. With such a frictional pairing it is preferable for the material with the better thermal conductivity, in this case SiC (with a thermal conductivity of 125 W/mK), to be used for the counter-ring, which then because it is integrated with the supporting component conducts the frictional heat produced in the axial face seal better than would be the case for a configuration in which the impregnated hard carbide (with a thermal conductivity of 25 W/mK) was responsible for conducting heat away from the housing.
According to a preferred embodiment of the invention the counter-ring is made in one piece with the housing, in which case the latter consists of aluminium or an aluminium alloy. Such a housing part can be especially well coated with a layer of tungsten carbide, because the temperature of the housing part during the coating process does not exceed 200xc2x0 C. Layer thicknesses of 0.1 mm to 1 mm can be achieved. By mechanical finishing, for example polishing, a surface roughness Ra of 0.06 mm can be obtained. The hardness of such a layer is 75-80 HRC.
According to another embodiment of the invention it is provided that the counter-ring is an integral component of the housing, i.e. is produced by working the surface of the housing itself. A prerequisite is that the cover plate be made of a material suitable for creating a seal in cooperation with the sliding ring. The counter-ring is then formed by processing an annular region of the cover plate in such a way as to achieve the required smooth surface.
According to the preferred embodiment of the invention, the sliding ring is provided with a groove on its bearing surface that cooperates with the counter-ring. This groove acts as an oil pocket. This interruption in the bearing surface of the sliding ring in effect produces two bearing surfaces one after the other, which increases the efficiency of the seal. Alternatively, the counter-ring could be provided with a groove. It is also possible to make the bearing surface of the counter-ring convex or concave, so as to allow for deformations under the action of heat.
An axial piston compressor in accordance with the invention offers the advantage that it is possible for the frictional heat that is generated to be especially well transferred from the counter-ring directly into the housing of the compressor. This reduces the warming of the axial face seal, which results in less thermal and mechanical distortion and deformation. Furthermore, installation is simplified because the counter-ring does not need to be separately inserted into the cover plate. Here the term xe2x80x9ccover platexe2x80x9d is understood to mean the part of the housing that is provided with the exit opening for the drive shaft; depending on the structural design and nature of mounting of the compressor, this part can also be the central part of the housing. Economical advantages are also obtained, because in comparison to the state of the art two parts are eliminated, namely the counter-ring as a separate component and the O-ring that would otherwise be needed as a seal for the counter-ring. If leakage should occur in the region of the axial face seal, a new counter-ring can be installed with little effort simply by exchanging the cover plate.
Advantageous embodiments of the invention will be evident from the subordinate claims.